body {
  background-image: url(http://api.xiaohigh.com/image/random);
  background-size: cover;
}
body .box:first-child {
  width: 500px;
  height: 100px;
  background: hsla(192, 60%, 44%, 0.6);
}
body .box:nth-child(2) {
  width: 500px;
  height: 100px;
  background: hsla(192, 60%, 44%, 0.7);
}
body .box:nth-child(3) {
  width: 500px;
  height: 100px;
  background: hsla(192, 60%, 44%, 0.8);
}
body .box:nth-child(4) {
  width: 500px;
  height: 100px;
  background: hsla(192, 60%, 44%, 0.9);
}
